1. Field of the Invention
The present invention relates to a cleaning process for photomasks, and especially to a cleaning process suitable for use upon cleaning photomasks with light-shielding coatings provided on surfaces of glass substrates.
2. Description of the Background Art
Photomasks with light-shielding coatings of a Cr or MoSi material provided on surfaces of glass substrates have been known for years. FIG. 4 is a flow chart of one example of a first conventional process for cleaning a photomask having the above-described construction.
As is illustrated in FIG. 4, according to the conventional process for cleaning the photomask, a cleaning step making use of sulfuric acid or sulfuric acid-hydrogen peroxide mixture is firstly conducted to eliminate organic substances adhered on a surface of the photomask (S100). The degree of contamination of a photomask with organic substances can be determined based on the wettability of the photomask with water or the like dropped onto its surface. Described specifically, the wettability of a photomask with water or the like dropped onto the photomask becomes better as the degree of contamination of the photomask with organic substances becomes lower. The objective of the cleaning step S100 can, therefore, be rephrased to improve the wettability of the photomask.
For rinsing off sulfuric acid or sulfuric acid-hydrogen peroxide mixture remaining on the photomask, a rinsing step is then conducted with pure water (S102). To increase the cleanliness of the photomask, it is necessary to perform thorough rinsing in this step.
To eliminate contamination adhering on the photomask, a cleaning step is then conducted using ammonia or hydrogen peroxide (S104). In this step, pure water or a cleaning solution prepared by adding a detergent to pure water may be used instead of ammonia or hydrogen peroxide.
As in the cleaning with sulfuric acid or sulfuric acid-hydrogen peroxide mixture, after the above-described step S104, a rinsing step making use of pure water is also conducted (S106). To sufficiently increase the cleanliness of the photomask, it is also necessary to perform thorough rinsing in this stage.
After the photomask is fully rinsed in the above-described step S106, a drying step is then conducted to dry the photomask (S108).
FIG. 5 is a flow chart of a second conventional process for cleaning a photomask having a Cr or MoSi light-shielding coating. According to the second conventional process, a cleaning step is firstly performed with a view to eliminating organic substances or improving wettability as in the first conventional process. Described specifically, a cleaning step is performed using sulfuric acid or sulfuric acid-hydrogen peroxide mixture as a cleaning solution (S110).
In the second conventional process, a cleaning step making use of an alkaline chemical such as aqueous ammonia or hydrogenated water is then performed to eliminate sulfuric acid or contamination still remaining on the photomask (S112).
Subsequent to the above-mentioned two steps, a drying step is performed to dry the photomask (S114).
According to the conventional cleaning processes, a photomask provided with a Cr or MoSi light-shielding coating is firstly cleaned with sulfuric acid or sulfuric acid-hydrogen peroxide mixture as mentioned above. In the first conventional process (FIG. 4), for example, insufficient performance of the rinsing step S102 may, therefore, leads to sulfate ions remaining on the surface of the photomask, thereby developing a potential problem that a blur tends to occur on the surface of the photomask. According to the first conventional process, a need has hence arisen to use a great deal of pure water of high temperature in the above-described step S102 to avoid such a problem.
Especially when a halftone photomask having an MoSi light-shielding coating is to be cleaned, the first and second conventional processes develop another problem as will be described next. In a halftone photomask, optical characteristics (transmittance and phase angle) of a light-shielding coating have high importance. Upon cleaning halftone masks, it is thus important to keep their optical characteristics unchanged. Keeping in step with the move toward mask patterns of increasingly shrunk sizes in recent years, however, variations in the optical characteristics of halftone masks, said variations arising as a result of cleaning with sulfuric acid or sulfuric acid-hydrogen peroxide mixture (S100 or S110), are becoming no longer ignorable.